Unit 5: Metallization

1. Introduction to Metallization

Metallization is the process of depositing thin films of metal onto a semiconductor wafer to form the conductive pathways that connect the various components (transistors, resistors, capacitors) of an integrated circuit (IC). These metal layers serve as the "wiring" of the chip, creating gates, contacts, and interconnects. The performance, reliability, and speed of an IC are critically dependent on the quality and properties of these metallization layers.

2. Desired Properties of Metallization

For a material to be suitable for metallization in semiconductor manufacturing, it must possess a specific set of electrical, chemical, physical, and process-related properties. No single material is perfect, so the choice often involves a trade-off based on the specific application (e.g., gate, contact, or interconnect).

2.1. Electrical Properties

• Low Resistivity: This is the most crucial property. Lower resistivity (ρ) reduces the resistance (R) of the interconnect lines (R = ρL/A, where L is length and A is cross-sectional area).

  • Impact: Lower resistance minimizes RC time delays, allowing for faster circuit switching speeds. It also reduces I²R power loss and voltage drops across the interconnects, improving power efficiency and signal integrity.
  • Examples: Copper (ρ ≈ 1.7 µΩ·cm), Aluminum (ρ ≈ 2.7 µΩ·cm), Gold (ρ ≈ 2.4 µΩ·cm), Tungsten (ρ ≈ 5.6 µΩ·cm).

• Good Ohmic Contact: The metal must form a low-resistance, non-rectifying (Ohmic) contact with both heavily doped n-type and p-type silicon.

  • Impact: A high-resistance contact (a Schottky contact) would act like a diode, impeding current flow and degrading transistor performance. Silicides (e.g., TiSi₂, CoSi₂) are often formed at the metal-silicon interface to achieve low contact resistance.

2.2. Reliability and Stability Properties

• High Electromigration Resistance: Electromigration is the transport of material caused by the gradual movement of ions in a conductor due to the momentum transfer from conducting electrons.

  • Impact: At high current densities, this "electron wind" can push metal atoms, leading to the formation of voids (open circuits) and hillocks (short circuits to adjacent lines). This is a major failure mechanism in ICs. Materials with higher melting points and stronger atomic bonds (like Copper and Tungsten) generally have better electromigration resistance than Aluminum.

• Thermal Stability: The metal film must be able to withstand the high temperatures of subsequent processing steps (e.g., dielectric deposition, annealing) without degrading.

  • Impact: The metal should not melt, agglomerate, or react with adjacent layers at processing temperatures (typically up to 400-450°C after metallization). This is why Tungsten (Melting Point ≈ 3422°C) is preferred over Aluminum (MP ≈ 660°C) for high-temperature contact applications.

• Chemical and Corrosion Resistance: The metal must be stable and not corrode or react with the surrounding chemicals during subsequent wet and dry etching processes.

  • Impact: Corrosion can lead to device failure. Gold is extremely inert but expensive and difficult to process. Copper is susceptible to oxidation and corrosion, requiring encapsulation with barrier layers.

2.3. Process and Mechanical Properties

• Good Adhesion: The metal film must adhere well to the underlying substrate, which is typically a dielectric like silicon dioxide (SiO₂) or a low-k material.

  • Impact: Poor adhesion can cause the film to peel or delaminate during subsequent processing or due to thermal stress, leading to device failure. Adhesion promoter layers, such as Titanium (Ti) or Tantalum (Ta), are often used.

• Ease of Patterning: The metal must be easily patterned using standard photolithography and etching techniques to define the intricate wiring patterns.

  • Impact: The ability to be anisotropically etched (i.e., etch vertically without significant lateral etching) is critical for creating fine, closely spaced lines. Aluminum is easily dry etched, while Copper is very difficult to dry etch, which led to the development of the Damascene process.

• Deposition Compatibility: It must be possible to deposit the metal as a high-quality, uniform thin film over the entire wafer with good control over thickness and properties.

  • Impact: The deposition method should provide good step coverage (the ability to conformally coat the vertical sidewalls and bottoms of trenches and vias) to avoid breaks in the conductive lines.

3. Metallization Choices for Different Layers

The choice of metal depends on its specific role within the IC structure.

4. Metallization Techniques

The primary methods for depositing metal films are Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). This section focuses on PVD, specifically vacuum evaporation.

4.1. Physical Vapor Deposition (PVD)

PVD encompasses a set of vacuum deposition methods where a material is transformed into its vapor phase in a vacuum environment and then condensed onto a substrate as a thin film. The atoms/molecules travel from the source to the substrate in a straight line-of-sight path. The two main PVD techniques are Vacuum Evaporation and Sputtering.

4.2. Vacuum Evaporation

Principle:
Vacuum evaporation is a PVD process where the material to be deposited (the "source" or "charge") is heated in a high-vacuum chamber until its vapor pressure becomes significant. This causes the material to evaporate (or sublimate). The resulting vapor-phase atoms travel through the vacuum and condense onto a cooler substrate, forming a thin film.

Key Requirement: High Vacuum
The process must be carried out at very low pressures (typically 10⁻⁶ to 10⁻⁷ Torr). This is crucial for two reasons:

1. Purity: It removes background gas molecules (like O₂, N₂, H₂O) that could react with the hot source material or co-deposit as impurities in the growing film.
2. Mean Free Path: It ensures a long mean free path (MFP), which is the average distance a vapor atom can travel before colliding with a background gas molecule. At high vacuum, the MFP is much longer than the distance between the source and the substrate, ensuring the atoms travel in a straight line without collisions, leading to a line-of-sight deposition.

// Relationship between Mean Free Path (λ) and Pressure (P)
λ ≈ (k_B * T) / (√2 * π * d² * P)

where:
k_B = Boltzmann constant
T = Temperature
d = Diameter of gas molecules
P = Pressure

As Pressure (P) decreases, Mean Free Path (λ) increases significantly.

4.3. Working of a Thermal Evaporator

A thermal evaporator is the simplest type of vacuum evaporation system. It uses resistive heating to evaporate the source material.

4.3.1. System Components

(Conceptual Diagram)

1. Vacuum Chamber: A sealed vessel, typically made of stainless steel or glass (a bell jar), that contains the entire setup.
2. Pumping System:
   • Roughing Pump (e.g., Rotary Vane Pump): Evacuates the chamber from atmospheric pressure down to a medium vacuum (~10⁻³ Torr).
   • High-Vacuum Pump (e.g., Turbomolecular or Diffusion Pump): Takes over from the roughing pump to achieve the required high vacuum (<10⁻⁶ Torr).
3. Pressure Gauges:
   • Low-Vacuum Gauge (e.g., Pirani Gauge): Measures pressure during the initial pump-down phase.
   • High-Vacuum Gauge (e.g., Ionization Gauge): Measures the very low pressures required for deposition.
4. Evaporation Source (Heating Element):
   • A refractory metal (high melting point) filament, boat, or basket that holds the source material.
   • Commonly made of Tungsten (W), Molybdenum (Mo), or Tantalum (Ta).
   • The shape (e.g., a "boat" with a dimple) is designed to contain the molten source material.
5. Power Supply: A high-current, low-voltage power supply connected to the evaporation source to pass a large current through it, causing resistive heating (Joule heating).
6. Substrate Holder: A fixture that holds the wafer(s) above the source, often with capabilities for rotation to improve film uniformity and heating to control film properties.
7. Shutter: A mechanical flag positioned between the source and the substrate. It is kept closed during the initial heating and stabilization of the source and is opened only when the desired evaporation rate is achieved.
8. Thickness Monitor:
   • Typically a Quartz Crystal Microbalance (QCM).
   • A thin quartz crystal disc oscillates at a stable resonant frequency. As material deposits on the crystal, its mass increases, causing the resonant frequency to decrease.
   • This change in frequency is precisely measured and correlated to the film thickness on the substrate.

4.3.2. Step-by-Step Process

1. Loading: The source material (e.g., pellets or wire of Aluminum) is placed into the tungsten boat. The clean substrate (wafer) is mounted onto the substrate holder. The chamber is sealed.
2. Pump-Down: The pumping system is activated. The roughing pump evacuates the chamber to ~10⁻³ Torr. Then, a valve is switched, and the high-vacuum pump further reduces the pressure to the target base pressure (e.g., 5 x 10⁻⁷ Torr). This can take from minutes to hours.
3. Source Heating: The power supply is turned on, and current is slowly increased to the filament/boat. The shutter remains closed. This initial heating phase serves two purposes:
   • Outgassing: It drives off any adsorbed gases and volatile contaminants from the source material and the chamber walls.
   • Melting: It melts the source material and stabilizes the evaporation rate.
4. Deposition: Once the evaporation rate is stable (as monitored by the QCM), the shutter is opened. The vapor atoms travel in a straight line from the source to the wafer, where they condense and form the film. The QCM provides real-time feedback on the growing film's thickness.
5. Termination: When the desired thickness is reached, the shutter is closed, and the power to the source is turned off.
6. Cool-Down and Venting: The system is allowed to cool down for a period. Then, the high-vacuum pump is isolated, and the chamber is slowly vented back to atmospheric pressure with an inert gas like Nitrogen (N₂). The chamber can then be opened, and the coated wafer is removed.

4.3.3. Advantages and Disadvantages of Thermal Evaporation

Advantages:

• Simplicity and Cost: It is a relatively simple and inexpensive deposition technique.
• High Purity Films: The high vacuum environment results in films with very low impurity levels.
• High Deposition Rates: Can achieve high deposition rates compared to some other techniques.
• Low Substrate Damage: The process is gentle and does not involve energetic ion bombardment (unlike sputtering), minimizing damage to the substrate.

Disadvantages:

• Poor Step Coverage: Due to the line-of-sight nature of the deposition, it is difficult to coat the vertical sidewalls of features like trenches and vias. The top surfaces receive much more material than the sidewalls, leading to potential discontinuities (shadowing effect).
• Limited Materials: Only suitable for materials that can be evaporated at temperatures below the melting point of the heating element (boat/filament). This excludes refractory metals with very high melting points.
• Alloy Deposition is Difficult: When evaporating an alloy, the constituent with the higher vapor pressure will evaporate more quickly, leading to a film composition that is different from the source material.
• Contamination from Source: The hot filament/boat can sometimes contaminate the film.